A boulder is a massive object that may be used for decoration in, for example, a front lawn of a residence. Like sized boulders, or stone blocks, may line a shoreline to minimize erosion. Massive rocks pose challenges to safety simply because of their weight.
As well as being massive, rocks are found in a variety of shapes. Some, like certain boulders, are egg shaped. Some, like stone blocks, are parallelepiped. The unique shape of a rock may render the rock relatively easy or difficult to manipulate between a thumb and a bucket.
Even if having a mass or shape that may pose few issues, rocks are carried from point to point over different terrain. A hydraulic excavator carrying a massive rock may have a flat easy route such as a driveway and sidewalk over which to carry the boulder or may encounter a tortuous undefined path in carrying the boulder to its final resting place on a shoreline.
Another feature of a boulder or stone block is its degree of hardness. For example, a granite block having quartz as part of its composition ranks high on the hardness scale. Blocks or boulders of such hardness, or of lesser hardness over a period of time, can significantly damage the frame of a thumb, even if the frame of the thumb is formed of hardened steel.